The invention resides in a method of producing polymer hollow fiber membranes, wherein molten polymer is conducted through an extruder in order to form the hollow fiber membranes.
Hollow fiber membranes of this type are used for the separation of material mixtures following a sieve-like separating mechanism. The type and size of pores formed in these membranes particularly of the pores at the membrane surface are important for the separation properties of the membranes. Because of a high porosity, the hollow fiber membranes have achieved a high permeability per volume unit.
In a known method of this type (U.S. Pat. No. 3,745,202), a mixture of polymers and plasticizers is extruded from the melt whereby a hollow fiber is formed which is stretched in a molten state and then cured in air, in a water bath or in an aqueous solution of a plasticizer. In this way, a porous hollow fiber is formed. The non-volatile plasticizer must subsequently be washed out. The actual membrane is then formed by the following treatment with hot water and, subsequently with an aqueous solution of dioxane, formic acid or a substance which is a solvent with respect to polymers. The additives remaining in the membrane must be removed at the end of the process in an expensive manner.
In addition, a multitude of other processes are known wherein hollow fiber membranes are produced by extrusion of polymers from a melt in a way similar to that disclosed in the process described before.
U.S. Pat. No. 3,873,653 discloses the manufacture of hollow fiber membranes from cellulose acetate, wherein dense hollow fibers are extruded from the melt and the actual membrane is formed subsequently by chemical treatment with formamide.
DE patent 28 33 493 discloses a method wherein a binary mixture of polypropylene and a monomer amine is extruded above the separation temperature and the membrane is formed by subsequent precipitation into a bath of the same amine below the separation temperature. Subsequently, the precipitation agent must be removed from the membrane. This known method, however, is disadvantageous in as much as the hollow fibers formed must be treated after the actual extrusion process in a more or less expensive physical or chemical after treatment steps in order to obtain the desired membrane properties. In these steps, a number of compounds such as solvents, plasticizers and others need to be removed in an expensive manner since the hollow fiber membrane would be unsuitable for most applications if these additional compounds would remain in the hollow fiber membrane.
It can be assumed that the known methods including the method described initially either require, for the production of the hollow fiber membrane, solvents which need to be removed in subsequent processing steps and of which small amounts remain in the finished membrane--which is disadvantageous--or that chemical substances are used as additives during the performance of the process such as plasticizers, pore formers etching chemicals, etc., which also need to be removed from the membrane by expensive procedures such as extraction, filtering and washing or which remain in the membrane as residues.
Furthermore, methods are known, wherein hollow fiber membranes are extruded from a melt of pure polymers without additional compounds. A known process of this type is described in JP-01 014 315. In this case, a thermoplastic polymer is extruded from the melt and is tempered at a temperature below the melting temperature for at least one minute in order to reach a crystallization degree of 25-50%. Subsequently, a number of cold stretching steps are performed below the melting temperature and a bottle-neck-type stretching is performed above the melting temperature and pores are formed. Similar additional processes are known, wherein pores are formed by thermal-mechanical post treatment in melt-extruded dense hollow fibers. JP-01 099 610 and JP 01 027 607, for example, disclose the manufacture of hollow fiber membranes of poly-(ethylene-co-chlorotrifluoroethylene) and U.S. Pat. No. 5,232,642 discloses the manufacture of hollow fiber membranes from polypropylene. The methods referred to above are different in the various thermal and mechanical process steps, which are adapted to the particular polymer. The duration of the heat treatment for the predetermined crystallization of the polymers varies between 1 minute and 30 minutes.
The known processes, including those referred to above for the manufacture of hollow fiber membranes by extrusion from a pure polymer melt after extrusion from the melt and subsequent thermal and mechanical after treatment, have the disadvantage that they can be used only for partially crystalline or crystallizable polymers and that it is difficult to achieve high spinning speeds because of the duration of the heat treatment.
It is therefore the object of the present invention to provide a process of the type referred to above with which, however, high spinning speeds of more than 2000 m/min can be achieved, wherein there is no need for additives or additional substances so that these substances do not need to be removed from the hollow fiber membranes in an expensive manner. The process further is to be simple and continuous.